Great streamers of stars loop around the disk of our Milky Way galaxy, stretching from pole to pole like ribbons on a birthday present. They were born from a violent collision between the Milky Way and a much smaller galaxy. Known as the Sagittarius Dwarf Elliptical Galaxy, it’s being ripped apart by the Milky Way’s gravity.

The galaxy’s center is only about 75,000 light-years away. But it’s veiled by thick clouds of dust in the Milky Way, so it wasn’t seen until 1994.

It’s only about one-tenth as wide as the Milky Way, and less than one-thousandth as massive. But it’s punched through the Milky Way twice in the last couple of billion years. Each encounter pulled out great streamers of stars from both galaxies. As the Sagittarius dwarf loops above and below the Milky Way’s disk, those streamers spread out to form galaxy-circling ribbons.

Over time, the Milky Way will incorporate the smaller galaxy’s stars into its own body, and the Sagittarius dwarf will disappear.

But in the meantime, the smaller galaxy is having a big impact on the Milky Way. It’s packed with invisible dark matter, so it’s quite dense. That allows it to blast through the Milky Way like a wrecking ball. It’s warping the outer edge of the galaxy’s disk like the brim of a hat. And one study shows that it’s responsible for creating one or more of the Milky Way’s spiral arms -- leaving a lasting impression on the galaxy that’s ripping it asunder.

More about companion galaxies tomorrow.

Script by Damond Benningfield, Copyright 2012

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